Subscriber line interface circuits are typically found in the central office exchange of a telecommunications network. A subscriber line interface circuit (SLIC) provides a communications interface between the digital switching network of a central office and an analog subscriber line. The analog subscriber line connects to a subscriber station or telephone instrument at a location remote from the central office exchange.
The analog subscriber line and subscriber equipment form a subscriber loop. The interface requirements of an SLIC result in the need to provide relatively high voltages and currents for control signaling with respect to the subscriber equipment on the subscriber loop. Voiceband communications are low voltage analog signals on the subscriber loop. Thus the SLIC must detect and transform low voltage analog signals into digital data for transmitting communications received from the subscriber equipment to the digital network. For bi-directional communication, the SLIC must also transform digital data received from the digital network into low voltage analog signals for transmission on the subscriber loop to the subscriber equipment. Strict gain and longitudinal balance control are required for subscriber loop applications.
In order to meet the strict requirements, high precision high voltage amplifiers are frequently used for processing voiceband signals. The voiceband output signal may be driven as a voltage through a desired output impedance to the subscriber equipment. Alternatively, the tip and ring lines of the subscriber loop are driven by currents corresponding to the voiceband signal while the desired output impedance is synthesized. Disadvantages of high precision high voltage amplifier solutions include the added cost and board area requirements for the SLIC.